1. Field of the Invention
The present invention relates to a process for preparing sorbitol by catalytic hydrogenation of suitable mono- and disaccharides.
2. Description of the Background
Sorbitol is prepared industrially by catalytic hydrogenation of glucose, fructose, sucrose or invert sugar (see H. Schiweck et al. “Sugar Alcohols” in Ullmann's Encyclopedia of Industrial Chemistry, 5th ed. on CD-ROM). For this purpose, the catalysts used to date have primarily been nickel catalysts, for example supported nickel catalysts or Raney nickel. However, there have also been several reports on the use of ruthenium-containing catalysts for this purpose. Generally, the ruthenium catalysts are supported catalysts that contain ruthenium on an oxidic support or an organic support such as carbon.
U.S. Pat. No. 4,380,680, U.S. Pat. No. 4,487,980, U.S. Pat. No. 4,413,152 and U.S. Pat. No. 4,471,144 describe the preparation of sorbitol by catalytic hydrogenation of glucose, in which catalysts are used that contain ruthenium on a support material which is stable under hydrothermal conditions. Hydrothermal support materials which are proposed are alpha-alumina (U.S. Pat. No. 4,380,680), titanium(IV) oxide (U.S. Pat. No. 4,487,980), titanium(IV)-halide-treated alumina (U.S. Pat. No. 4,413,152) and theta-alumina (U.S. Pat. No. 4,471,144).
U.S. Pat. No. 4,503,274 discloses catalysts for hydrogenating glucose to sorbitol which are prepared by impregnating a hydrothermally stable support with an aqueous ruthenium halide solution and subsequently hydrogenating the solid at from 100 to 300° C.
U.S. Pat. No. 3,963,788 describes hydrogenating corn starch hydrolyzates to sorbitol in the presence of ruthenium catalysts in which the ruthenium was supported by an aluminosilicate-based specific zeolite. U.S. Pat. No. 3,963,789 proposes as support for ruthenium catalysts crystalline aluminosilicate clays, in particular montmorillonite.
FR-A 2526782 describes the use of a ruthenium chloride prepared by reacting sodium chloride and ruthenium via Na2RuCl6 for preparing silica-supported ruthenium catalysts for hydrogenating mono- and oligosaccharides, for example for preparing sorbitol.
The processes known from the prior art for preparing sorbitol by hydrogenation in the presence of ruthenium catalysts, owing to the only moderate activity of the catalysts, give sorbitol only with moderate space-time yields, based on the catalyst used. In view of the high costs of ruthenium, therefore, the economic efficiency of these processes leaves something to be desired. In addition, the selectivities of the catalysts are not sufficient, so that additional expenditure is required in isolating the products of value. In particular, epimerization of the hydroxyl groups is frequently observed.